This invention generally relates to medical devices, and particularly to balloon catheters.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient""s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter, and the stent left in place within the artery at the site of the dilated lesion.
In the design of balloon catheters, catheter characteristics such as flexibility, pushability, and profile must be tailored to provide optimal performance for a particular application. Angioplasty balloon catheters are preferably flexible yet pushable and with a low profile for improved ability to track the tortuous anatomy and cross lesions with the balloon in the uninflated state. A distal section with a low profile, i.e., a small leading outer diameter, is necessary to guide the catheter to the location of the lesion with as little damage to the patient""s body lumen as possible. During balloon manufacture, a polymeric tube is formed into a finished balloon having a desired outer diameter and length. The skirt sections of the balloon are then bonded to the shaft. The manufacturing process may create a step increase in outer diameter from the catheter distal tip to the balloon, or an increase in catheter stiffness at the bond between the balloon and the shaft, which consequently may create a higher profile and cause damage and difficulty moving the catheter through the body lumen. When extruded tubing with a constant inner and outer diameter is blow molded to form a balloon, the balloon typically has a thin working length and a disadvantageously thick skirt section. This is due to the fact that the balloon working length has a blow-up-ratio (BUR) (i.e., balloon outer diameter divided by the tubing inner diameter) which is higher than the BUR of the skirt sections.
What has been needed is an improved method of forming a catheter balloon having a thin walled skirt section.
This invention is directed to a method of forming a catheter component, such as a balloon for a catheter, in which the component has a thinned skirt section formed by expanding tubing to a radially enlarged outer diameter and then decreasing the inner diameter and the outer diameter of the radially enlarged skirt section before or during bonding of the skirt section to a section of the catheter shaft. While discussed below primarily in terms of a balloon, it should be understood that other catheter components such as soft distal tips and shaft sections can be formed with a thinned end section using the method of the invention. The method generally includes blow molding tubing in a balloon mold to form the balloon, the balloon mold having a skirt portion for forming the radially enlarged skirt section, and having an interior surface which corresponds to the desired form of the working length and tapered sections of the balloon. The skirt portion of the balloon mold has an inner diameter which is larger than an outer diameter of the thinned skirt section of the balloon, so that the balloon mold produces a balloon having a radially enlarged skirt section with a larger outer diameter than is desired for the finished balloon. Thus, by first expanding the tubing in the balloon mold, and then decreasing the inner and outer diameter of the radially enlarged skirt section, the resulting skirt section of the balloon has a thinned skirt section which provides a balloon catheter having improved flexibility and low profile.
Balloons of the invention, as with conventional catheter balloons such as angioplasty or stent delivery balloons, typically have a working length configured to expand to perform a procedure such as dilatation of a lesion or implantation of a stent. The balloon has a proximal tapered section at a proximal end of the working length, a proximal skirt section at a proximal end of the proximal tapered section for securing the balloon to a catheter shaft, a distal tapered section at a distal end of the working length, and a distal skirt section at a distal end of the distal tapered section for securing the balloon to a catheter shaft. The expanded outer diameter of the radially enlarged skirt section is larger than the outer diameter of the end of the tapered section adjacent thereto. In a conventional balloon mold, the mold interior portion which forms a skirt section of the balloon has an inner diameter which is the same as or smaller than the inner diameter of the adjacent end of the adjacent tapered section. As a result of expanding the skirt section to a larger diameter than in a conventional mold, the skirt section formed according to the method of the invention has a thinner wall thickness than would otherwise be produced by a balloon blow molded in a conventional balloon mold. In one embodiment, a balloon of the invention having a 3.0 mm nominal inflated outer diameter has a thinned distal skirt section wall thickness of less than about 0.036 mm to about 0.04 mm, and a balloon having a 5.0 mm nominal inflated outer diameter has a thinned distal skirt section wall thickness of less than about 0.11 mm to about 0.13 mm, and, specifically, a thinned wall thickness of about 0.08 mm to about 0.09 mm.
In a presently preferred embodiment, the inner and outer diameter of the radially enlarged skirt section is decreased by necking the radially enlarged skirt section before bonding to the catheter shaft. The radially enlarged skirt section is preferably necked by applying heat and axial tension to the radially enlarged skirt section to thereby decrease the inner and outer diameter and further decrease the wall thickness of the skirt section. The necked skirt section is then fusion or adhesively bonded to the catheter shaft. In the embodiment in which the necked skirt is fusion bonded to the shaft, the wall thickness of the skirt section is further decreased by causing the skirt section polymeric material to flow during the fusion bonding. In a presently preferred embodiment, the balloon tubing is expanded in the balloon mold so that the radially enlarged skirt section has an inner and outer diameter greater than an inner and outer diameter of an intermediate section located between the radially enlarged section and the tapered section adjacent thereto. The smaller inner diameter of the intermediate section is preferably configured to facilitate gripping of the balloon onto a mandrel positioned in the balloon during necking of the radially enlarged skirt section.
Catheter balloons, and particularly angioplasty and stent delivery balloons, having thin skirt sections provide higher flexibility than balloons with thick skirt sections, and therefore provide easier access to a lesion in a blood vessel. A thin skirt is especially critical at the distal skirt which comes into contact with the lesion first. The method of the invention may be used to form the proximal and/or the distal skirt section of the balloon with a thinned wall thickness. However, in a presently preferred embodiment, at least the distal skirt section is formed with a thinned wall thickness according to the method of the invention, in order to provide improved flexibility and low profile at the distal end of the catheter for improved ability to cross a lesion.
The balloons of the invention may be used on a variety of catheter types, including dilatation catheters, such as angioplasty catheters, and stent delivery catheters, and the like. A presently preferred embodiment of the invention is a peripheral balloon catheter. In one embodiment, the peripheral balloon catheter has a balloon with an inflated working outer diameter of at least 4 mm, and preferably about 5 mm to about 10 mm. Peripheral balloon catheters typically have balloons with a larger outer diameter than coronary balloon catheters, for use in large peripheral blood vessels. Such large balloons generally have skirt sections with a large wall thickness unless thinned before or after blow molding of the balloon. Necking procedures which thin the skirt section by axially stretching the balloon tubing, typically before blow molding of the balloon, produce disadvantageous orientation in the polymer molecules. As the degree of orientation increases, it becomes more difficult to expand the pre-necked portion of the balloon tubing. Therefore, the amount of skirt thinning possible from pre-necking the balloon tubing is limited. The method of the invention thins the skirt section in part by first expanding the skirt section to the radially enlarged diameter. Consequently, unlike pre-necking, the method of the invention results in thinned balloon skirt sections without disadvantageously axially orienting the polymer molecules, and thus provides for optimum skirt thinning.
Although discussed primarily in terms forming a balloon for a balloon catheter, the invention should be understood to include other components for medical devices such as a catheter distal tip or shaft, in which thinning a section of the component is desired.